The Dbl oncogene product was originally identified as the transforming gene from human diffuse B- cell lymphoma. It is now evident that Dbl is only one member of a growing family of cellular proteins that contain a unique structural 'signature': the tandem arrangement of pleckstrin homology (PH) and Dbl homology (DH) domains. Mutated versions of most Dbl-like genes were identified as causing a variety of human malignant and invasive pathologies, suggesting a key role for these molecules in regulation of normal growth. To date, only one biochemical activity has been associated with Dbl-related proteins, namely to serve activators of (cause GTP binding to) low Mw GTP-binding proteins from the Rho-subfamily, such as Rho, Cdc42 and Rac. It is generally hypothesized that the basis for the oncogenic capability exhibited by Dbl molecules is an outcome of their ability to cause GTPase activation. In accordance with this hypothesis, activated alleles of Cdc42, Rac and Rho proteins were demonstrated to possess some growth- regulatory properties. The magnitude and extent of GTPase-induced proliferation, however, has been consistently much weaker than that observed upon transformation by Dbl-like proteins. The proposed research aims at gaining a better understanding of the molecular mechanisms that underlie Dbl-induced transformation of mammalian cells. Four specific avenues of investigation will be pursued which constitute the specific aims of this proposal. These are: 1). To 'reconstitute' Dbl-transformation in a mouse fibroblast system using novel activated GTPases and, if required, other proteins that we will identify. 2). Purify, identify and clone a target of Cdc42 that is likely to mediate the Dbl-transformation signal. 3). Understand how Dbl is regulated in normal (non-transformed) cells, and 4). To find the molecular basis of Dbl-induced activation of S6 kinase (a key regulator of protein synthesis) and its role in Dbl-transformation. We anticipate that answering these questions will enable us to dissect Dbl- signaling into molecular components which will result in better understanding of the basic mechanisms that regulate specific aspects of cell growth. Additionally, the results of these studies will potentially allow for developing specific intervention strategies for Dbl-induced malignancies.